1. Field of the Invention
The present invention relates to Atomic Force Microscopy (AFM) and other nanotechnology apparatus using microcantilevers as force sensors. More particularly, the invention relates to an apparatus and method for measuring the deflection of a microcantilever due to stress and to compensate for this stress in the microcantilever.
2. Description of the Related Art
For many applications, a microcantilever is the most sensitive force detector available in industry. In recent years, advances in micro-electromechanical device (MEMS) manufacturing techniques have made the microcantilever a mass producible device. The availability of these devices has enabled the widespread use of scanning probe microscopes, such as the atomic force microscope (AFM), across many industries. For example, microcantilevers are now being exploited as sensors for airborne reagents, as well as reagents in solution.
Microcantilevers are simply small beams ranging in length from a few millimeters down to a few microns. They are typically batch fabricated by microstructure processing (MSP) from semiconductor wafers. More particularly, bulk processing of silicon and surface processing are used to produce silicon microcantilevers and silicon nitride or poly-silicon cantilevers.
Microcantilevers can have the shape of a simple beam extending from a larger support member, or they can have a more complex shape. Many AFM microcantilevers have a two-beam geometry and form a V-shape with the apex at the distal end where a probe tip is mounted. Another geometry of AFM microcantilevers comprises a diffraction grating, where more than one microcantilever has finer finger elements that are interleaved with fingers of another microcantilever to form an optical grating structure.
The above-mentioned microcantilevers function like a force sensor, similar to a small spring. An applied force is measured generally linearly as the microcantilever is deflected. Alternatively, it is useful to use the resonant response of the microcantilever to measure the applied force as a function of a change in the natural frequency of the microcantilever or, for example, as a function of a change in the quality factor of the natural resonance of the microcantilever when the microcantilever touches a surface. Measuring the change in the microcantilever resonance is a useful mechanism for detecting a change in the force on the microcantilever.
As the applied force causes a deflection of the microcantilever sensor, it becomes necessary to measure the microcantilever deflection with great precision. Among the deflection detection schemes that have proved satisfactory are optical beam bounce, laser and white light interferometry, laser diode feedback, capacitance C measurement, tunneling current measurement, polarization detection, interdigital diffraction grating techniques, and piezoresistance measurement. Each of these detection schemes, with the exception of piezoresistance, requires that the microcantilever be aligned with, or placed in proximity to the deflection detection device. For example, optical beam bounce is an optical based system that typically includes a laser and a photodetector which interact with the microcantilever to measure deflection of the cantilever. A position-sensitive detector (PSD) receives a reflected beam and generates a signal indicative of the beam's changing position on the detector, as described in further detail below. In addition to its high sensitivity, this scheme has proven to be well suited for measurements in an ultra-high vacuum (UHV) environment.
Another detection scheme uses laser and white light interferometry, in which a laser beam is reflected off the microcantilever and a photodetector is used to sense the angular movement of the beam as the microcantilever is deflected. Still another way of measuring microcantilever deflection is using a tunneling current detector. Tunneling occurs between an auxiliary conducting tip and the microcantilever, which are separated by several angstroms. The application of a bias voltage produces a tunneling current through the air gap separating the microcantilever and the tunneling tip from one another by modulating the tunneling current, it is possible to detect deflections of the microcantilever as small as 10−4 angstroms. Notably, deflection of the microcantilever will vary the gap between the tunneling tip and the microcantilever and produce a change in the tunneling current.
Minute deflections of a microcantilever can also be monitored by a change in capacitance between the microcantilever and a reference plate. A high-Q resonant circuit is employed to measure the changes in capacitance. On the slope of the electronic resonance of the circuit, small resonance frequency variations caused by deflection of the microcantilever result in an amplitude change in the electronic resonance. This amplitude change provides a very sensitive measure of the microcantilever deflection.
In a basic homodyne microcantilever deflection system, a polarized laser beam passes through a beam splitter and is incident on a Fabry-Perot etalon whose reflecting surfaces consist of an optical flat and the microcantilever. The beam, which is reflected back from the etalon is incident on the same beam splitter and is deflected into a photodetector. The photodetector then generates a photocurrent used to measure the deflection of the microcantilever.
In a differential homodyne microcantilever deflection system, a fraction of the laser beam (serving as a reference beam) is diverted by a beam splitter to a first photodetector. The light passing through the beam splitter (serving as a signal beam) is incident on the Fabry-Perot etalon, reflected back, and deflected towards a second photodetector. Next, the currents of the two photodetectors are compared, and their difference yields a signal that is used to measure the deflection of the microcantilever. Homodyne detection may also be implemented using a fiber-coupled laser.
In a heterodyne detection system, a first beam splitter divides a laser beam into two beam components. One beam component is passed through an acousto-optic modulator that shifts the beam frequency by some amount (omega) and the other beam component is reflected onto a mirror as a reference beam. The beam with the shifted frequency, which serves as the signal beam, passes through a polarizing beam splitter, a quarter-wave plate, and finally through a lens that focuses it onto the microcantilever. The microcantilever reflects the beam back through the lens and quarter-wave plate, which rotates the polarization of the beam on the two passes by 90 degrees. The polarizing beam splitter then reflects the beam incident on the photodetector. The reference and signal beams interfere on the photodetector, which generates a current consisting of a spectrum of frequencies. The photocurrent is then fed into a single side-band receiver driving a phase-sensitive detector that provides a signal representative of the microcantilever deflection.
The sensitivity of solid-state lasers to optical feedback is often used in AFMs as a detection means for microcantilever deflection. Under certain optical feedback conditions, the operation of the laser can become noisy, bi-stable, or chaotic. A microcantilever positioned several microns in front of the laser output can induce variations in this optical feedback. The lever and front facet combination act as a lossy Fabry-Perot etalon, whose reflectivity serves as the effective reflectivity of the front facet of the laser. The optical losses are the result of the diffraction effects where successive reflections between the front facet and the lever decrease for higher orders. This system is simple to assemble from a few components. However, the system's sensitivity relies greatly on the precise positioning of the microcantilever with respect to the front facet of the laser.
A polarization detection system differs from other optical detection systems in that the measured deflection of the microcantilever sensor is first converted into a polarization signal prior to being converted into an amplitude signal. The conversion from a polarization signal to an amplitude signal is accomplished with two polarizing prisms rotated forty-five degrees relative to each other that produce an interference between s- and p-polarized fields. The output from this differential system has a common-mode rejection that cancels laser noise. This output is used to measure the deflection of the microcantilever.
Another microcantilever deflection detection system becoming more widely used is an interdigital diffraction grating disposed on the microcantilever. Fingers protrude from the microcantilever either perpendicularly or longitudinally. These fingers are interleaved with fingers protruding from either (1) another proximal microcantilever, or (2) the microcantilever substrate. The interleaving fingers form a grating structure whose spacing changes when the microcantilever is deflected. A laser beam incident on the grating structure produces an interference pattern on a photodiode positioned to collect laser light reflected from the grating structure.
As with each of the previously described systems, the positioning of the grating structure with respect to the laser and photodiode is critical to the sensitivity of the system. Moreover, the output of the photodetector varies sinusoidally with the deflection of the microcantilever. Correct positioning of the microcantilever may require that the phase of the sinusoidal output be positioned at a predetermined value.
The optical beam-bounce deflection detection system is the most commonly used system in AFM for microcantilever deflection detection. A collimated laser beam is focused on the microcantilever and is reflected back towards two closely spaced photodetectors (e.g., bi-cell) whose photocurrents are fed into a differential amplifier. A minute deflection of the microcantilever causes one photodetector to detect more light than the other, and the output of the differential amplifier is directly proportional to the deflection of the microcantilever. This system requires precise angular as well as positional alignment of the microcantilever with respect to the incident laser beam such that the reflected beam is directed precisely to the junction between the two photodetectors.
As the use of microcantilevers becomes more widespread, compound microcantilever structures are becoming commonplace. For example, microcantilevers are often fabricated in multilayer structures to cause bimorphic deflection bending of the microcantilever in response to a stimulus. Unfortunately, compound microcantilevers further complicate ensuring proper alignment within the corresponding detection scheme.
In particular, during process, layers of specialized materials are often coated onto the surface of the microcantilever. The most common microcantilever coating is an aluminum or gold coating on one side of the AFM microcantilever to increase optical reflectivity for the purpose of beam bounce deflection detection. In another example, magnetic force imaging with an AFM is accomplished by coating the tip side of the AFM microcantilever with a ferromagnetic metal that will render the cantilever sensitive to a force in the proximity of a magnetic field. Metal coatings are also used to prepare the microcantilever surface for biochemical reactions, or for absorption of mercury vapor or other reagents. Polymeric coatings are being developed for the purpose of detecting the presence of reagents through absorption. When the polymer absorbs the target, it typically swells, increasing the mass of the microcantilever or bending it due to increased stress.
Overall, these coatings are controlled to minimize the surface stress on the microcantilever. Nevertheless, surface stress still often causes undesirable bending. It is difficult to control the stress in these coatings, so typically the opposite side of the microcantilever is coated in the exact same manner to attempt to compensate for the stress induced bending. However, in addition to being a wasteful process, coating both sides of the lever produces only marginal results in compensating for coating stress.
The present invention is directed to compensating for stress, such as coating stress, and is described in the context of a multi-layer microcantilever structure known as a compound piezoelectric microcantilever. This recent advance in AFM has enabled a factor of ten-fold speed increase in image acquisition and is described in detail in U.S. Pat. No. 6,189,374 issued to the assignee of the present invention. Most generally, the cantilever described herein is a microcantilever with an integrated piezoelectric layer. The piezoelectric material, preferably zinc oxide (ZnO) or lead zirconium titanate (PZT) is disposed on the microcantilever between two metal electrodes. An applied voltage across the electrodes causes an expansion or a contraction of the piezoelectric layer, which causes a stress in the microcantilever. This stress, in turn causes a bending motion of the microcantilever in which the distal end moves through a small arc. The movement of such a microcantilever for AFM allows following the topography of the sample under investigation with a much higher bandwidth than was previously available.
A common problem in all microprobe structures having such a microactuator is the manifestation of interlayer stress inducing static bending of the microcantilever. For many of the above examples, the deflection or bending of the microcantilever is the desired result of the measurement; however, this deflection typically is small enough to remain within the sensitive range of, for example, the detector. The problem of stress arises in the manufacturing of the multi-layer microcantilevers when the deflection due to stress is large enough to move the microcantilever outside the sensitive range of the deflection detection system.
Until now, stress deflection of the microcantilever has been tolerated in the R&D environment by time-consuming adjustment of an AFM apparatus. However, it is no longer acceptable in a commercial market to adjust the apparatus to accommodate poor manufacturing control of the microcantilever sensor. Most often, stress cannot be entirely overcome by engineering or manufacturing control, or by coating both sides of the cantilever, as described previously. As a result, the process is considered low yield and a significant volume of product is discarded as unusable.
In view of the above-described problems relating to stress deflection in compound microcantilevers, the art of microcantilever fabrication was in need of an apparatus and method for, among other things, overcoming the problem of static bending in microcantilevers, especially in the case of volume production.